1. Field of the Invention
The present invention relates to superconducting material and a method for preparing the same. More particularly, it relates to a novel superconductor which exhibits superconducting property at a very high temperature and a method for preparing the same.
2. Description of the related art
Certain kinds of materials exhibit the perfect diamagnetism under the phenomenon of superconductivity and no electrical resistance is observed even if a definite constant current flows therethrough.
Such phenomenon of superconductivity is applicable to various industrial fields. For example, in the field of power electric, MHD power generation, power transmission, electric power reservation or the like are expected. In the field of transportation, magnetic levitation trains, magnetically propelling ships or the like are expected. In the field of measurement, high sensitive sensors or detectors for sensing a very weak magnetic field, microwave, radiant ray or the like are expected. In the medical field, high-energy beam radiation units or the like are expected. In the field of science, NMR, high-energy beam radiation units or the like are expected. In addition to the abovementioned power electric applications, the superconducting materials are expected in the field of electronics. For example, a Josephson device which is an indispensable switching device for realizing a high-speed computer which consumes very reduced power is expected to be realized.
However, their actual usage have been restricted because the phenomenon of superconductivity was observed only at very low cryogenic temperatures. Among known superconducting materials, a group of materials having so-called A-15 structure show rather higher Tc (critical temperature of superconductivity) than others, but even the top record of Tc in the case of Nb.sub.3 Ge which showed the highest Tc could not exceed 23.2K at most. This means that liquidized helium (boiling point of 4.2 K.) is only one cryogen which can realize such very low temperature of Tc. However, helium is not only a limited costly resource but also require a large-scaled system for liquefaction. Therefore, there had been a strong demand for another superconducting material having higher Tc.
The possibility of an existence of new types of superconducting materials having much higher Tc was revealed by Bednorz and Muller, who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 189]
The new type compound oxide superconductor discovered by Bednorz and Muller is represented by [La, Sr].sub.2 CuO.sub.4 which is called the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxides. The K.sub.2 NiF.sub.4 -type compound oxides show such higher Tc as 30 K. which are extremely higher than known superconducting materials.
It was also reported that C. W. Chu et al. discovered, in the United States of America, another superconducting material so called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having the critical temperature of about 90 K in February 1987 (Physical Review Letter (58) 908-910).
The discovery of such high-temperature superconductors is expected to accelerate the utilization of superconducting technology. In fact, in the case of the above-mentioned new type superconductors, the superconductivity can be realized in liquidized nitrogen, as a cryogen, which is cheaply available on market and hence the cost performance of the superconducting technology can be improved.
However, it is still requested to elevate the practical critical temperature in order that the superconductor can be used in a stable and confidential condition. In other words, it is desirable to increase the discrepancy between the critical temperature Tc of a superconductor and the cryogen temperature (particularly a boiling temperature of the cryogen).
An object of the present invention is to provide a novel superconducting material which exhibit the superconducting property at a higher temperature and which can lighten a burden on the liquefaction system of cryogen, and a process for producing the same.